Low-molecular-weight nematic compounds, which are widely used in liquid-crystal displays (LCDs), are distinguished by good alignability and switchability in an electric field. However, their aligned structure is not particularly stable without an electric field. With increasing warming, the nematic alignment decreases until finally an ordered structure is no longer present. By contrast, some applications, for example polarizing filters and retardation films, require materials whose nematic alignment is retained over a broad temperature range. In many cases, it is also preferred for the aligned structure to be fixed in a solid film instead of, as in the case of low-molecular-weight nematic compounds, for a liquid substance to be enclosed between substrates.
A known way of fixing liquid-crystalline properties is the binding of the liquid crystals into polymer networks, for example by subsequent crosslinking of aligned liquid-crystalline side-chain polymers. However, liquid-crystalline side-chain polymers are generally more difficult to align than low-molecular-weight liquid crystals owing to their higher viscosity, which generally increases with increasing molecular weight. Cholesteric siloxanes are examples of liquid-crystalline side-chain polymers which can be easily aligned . U.S. Pat. No. 5,211,877 describes easily aligned compounds based on cyclic organosiloxanes having side chains containing cholesterol and methacryloyl groups. However, replacement of cholesterol by a compound containing two phenyl radicals only induces a material having a smectic SA phase and secondary amounts of a more highly ordered SB phase. This material is difficult to align and is therefore unsuitable for the proposed applications.
U.S. Pat. No. 5,610,258 discloses that according to the art, neither organosiloxanes having various types of side groups, each containing two aromatic or cycloaliphatic radicals, nor organosiloxanes having identical mesogenic side groups each containing more than two aromatic or cycloaliphatic radicals, are easy to align and at the same time exhibit a broad nematic phase. In contrast, the patent discloses cyclic organosiloxanes having at least three different mesogenic side groups per organosiloxane, where at least two mesogenic side groups per organosiloxane contain at least three aromatic, heteroaromatic and/or cycloaliphatic radicals. These organosiloxanes are said to be readily alignable side-chain polymers having a broad nematic phase. The synthesis of the compounds described proceeds via a large number of separate steps and is too complex for an industrial process. Production of optically anisotropic layers from these materials, to be complete within an acceptable time in production terms, produces only hazy, optically anisotropic layers.